So many people ask if the DAT is harder than the MCAT, and geez, if you could take the MCAT a week before and end up with those scores on a practice test... That's saying something! The tests are very different, though, and I think the DAT sciences may very well be more difficult (in my unprofessional opinion)! It kind of rubs me wrong when people think of going to dental school as the "easy route." It ain't easy!!
I also remember reading a thread back in the day where a person said they studied a few weeks for the MCAT and did well but it took them months to study for the DAT and do well. I wonder if this is at all common between those who take both tests? The amount of people who do this is probably far too small to form any consensus.
Congratulations, and best of luck in all that you do!
P.S. I don't think that taking both will necessarily hinder you too much, provided that you have a good reason for pursuing dentistry instead of medicine--and this would take some time and experience to determine. If you're applying this year, though, it might seem like you are just completely undecided, and schools might not take too fondly to that. If you are a 2013 applicant, though, then I think this is fine! It's good to keep options open!
P.S.S. It isn't any of my business, but when you get your MCAT scores, I'd love to know what they were... I've just always been curious as to how a person's scores would compare between the two. If you'd rather not, though, I completely understand!